Market Penetration is a term used to describe how a product/service performs in terms of sales relative to the overall sales of that product/service in the market. This is a good indicator for a business to understand how much of the market share they have attained, and is also a great KPI by which to measure performance growth.
If you sold one thousand sunbeds in the UK in 2022 when ten thousand sunbeds were sold that year, that indicates your market share, and you would increase that with Market Penetration to sell a great portion of that sunbeds sold in 2023. This is very specific to your target market (in this case, people who want sunbeds in the UK) and how much of the product/service is sold across the entire market.